Mixed materials in speakers chassis

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This section presents the theories behind the choice of construction materials for the Atomica Floor and Book 2 loudspeakers: speakers whose cabinets are based on the contrast between poplar plywood and glass.

Poplar

The planks used in our production are made from a particularly compact European variety, with elasticity and compactness similar to harder woods. However, it maintains the characteristic lightness of the poplar, which – being voracious for water with a soil absorption of up to 2,000 litres per day – produces resistant and light wood once dried. The poplar used in the production averages 720 Kg/m3 at a humidity between 7 and 12%. We are dealing with a material whose stiffness/weight ratio is excellent, to say the least.

For those who believe that this choice is based on economic or logistical reasons, I would like to point out that: poplar plywood has a cost (in the thicknesses we use it in) that is only slightly lower than birch plywood. Its milling is more complicated than birch plywood. It is much more complicated than MDF, whose finish is virtually unaffected by the milling cutters in use, the feed speeds, and the spindle rotation. This is because MDF is not wood, but a homogeneous agglomerate of finely shredded, fibre-free processing waste, which is easy to process both during milling and manual finishing.

Glass

Another burning topic in the field of loudspeakers. It is unthinkable to entirely use a material with such a high mass in a frame. However, it’s quite reasonable to use it as a damping element in a lightweight wooden cabinet.

Poplar + glass

We now approach the coupling of two materials whose characteristics are – to say the least – opposite. But before talking about this, we need a small introduction to the physical phenomenon of resonance:

1- We speak of mechanical resonance when a mechanical system is excited at a frequency compatible with the characteristic oscillation of the system itself, and it amplifies it. This phenomenon causes an accumulation of energy within the system, which is counteracted by the damping coefficient.

2- Acoustically speaking, a range of vibrations are generated inside a resonator (the cabinet) which are characterised by its geometric and mechanical properties. The resonance phenomenon involves both the resonator and the vibrating element (the loudspeaker).

3- A vibration that is triggered in a system made up of differentiated geometries and sections is called parcellized. These geometries and sections vibrate simultaneously and separately giving rise to a complex motion (multiple harmonic motions).

It is now clear why glass alone cannot optimally contribute to the composition of a cabinet: although it is an exceptionally hard material, its high specific weight and low damping cause considerable energy accumulation in the system (see 1).

As for the wooden frame, which will house the loudspeakers, a considerable effort has been made in the Atomica loudspeakers to minimize energy exchanges between the parts, by using carefully studied geometries and shapes (see 2). For further information, please refer to the article “Elements for cabinet design”.

The above-mentioned frame is therefore characterised by high stiffness and very low elasticity and mass. A perfect element, were it not for its poor damping properties. And this is why glass is incorporated into the frame.

The combination of these two materials is achieved by interposing a special semi-expanded seal, through which the glass is bolted against the wood at a specific – electronically measured – pressure. Besides giving the glass the necessary damping, this procedure allows the wooden frame to gain the necessary damping mass.

The resulting cabinet takes due account of points 1-2-3. Parcellation is best exploited because two materials that are completely incompatible in terms of hardness, elasticity, and mass are coupled in such a way as to mutually dampen the resonance motions and delocalise them into the frequency domain.

The following charts show the harmonic distortion detected on Atomica Book 2 and Atomica Book Fraxinus respectively. The latter differs only in the side walls, which are made of solid ash instead of glass.

Both charts show a preponderance of second and then third harmonics, highlighting a pleasant characterization. The total average (upper line) is very close in both models. Note, however, its trend and also that of the distinct harmonics: in the glass model, we can appreciate an exceptionally constant and linear trend, free of perturbations, and this shows that this type of cabinet provides superior performance, better damping, and greater naturalness compared to its counterpart built with traditional materials.

REFERENCES

  • Ahnert W, Steffen F, “Sound Reinforcement Engineering”, 1999.
  • Arnol’d V, “Mathematical Methods of Classical Mechanics”, 1999.
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